The present invention relates to a process for the preparation of phosgene by the exothermic reaction of chlorine with carbon monoxide in the presence of an activated carbon catalyst. More particularly, the present invention relates to a process for the preparation of phosgene by the reaction of chlorine and carbon monoxide in which both raw materials are efficiently utilized and in which the amount of unreacted raw material, particularly carbon monoxide, discarded as a waste material is minimized.
The basic process for producing phosgene by the exothermic reaction of chlorine with carbon monoxide in the presence of a carbon catalyst is well-known in the art. In accordance with the well-known process, a vapor phase mixture of chlorine and carbon monoxide is introduced into a reactor containing an activated carbon catalyst whereupon the chlorine and carbon monoxide react to form phosgene as shown by the equation: EQU CO+Cl.sub.2 .revreaction.COCl.sub.2 +25.7 K Cal./mole
In most commercial applications, two reactors are used in series. In the first or "primary" reactor, the major portion of the reactants are converted to phosgene; this is followed by the second or "finishing" reactor where the residual unreacted components are further reacted to achieve substantially complete conversion of the reactants to phosgene.
It is generally desirable to convert as much of the chlorine feed to phosgene as is possible and to minimize the amount of residual chlorine in the reaction product. This is generally accomplished by maintaining a stoichiometric excess of carbon monoxide in the reactor to drive the reaction towards completion with respect to chlorine.
Phosgene is recovered from the product gases by passing the gases through a cooling system where they are cooled sufficiently to cause the phosgene contained therein to condense out. The condensed phosgene is then recovered as a liquid product (usually under pressure).
The noncondensed product gases, containing unreacted carbon monoxide (i.e., the excess carbon monoxide that was fed to the reactor and which passed through it unreacted), chlorine, hydrogen chloride, and other by-products as well as a small amount of phosgene, are then passed through an absorption system where the hydrogen chloride and phosgene are neutralized by a dilute caustic solution. The product gases, minus the hydrogen chloride and phosgene which were absorbed in the absorption system, are vented from the absorption system to an incinerator where they are burned.
Although this basic process is very efficient with respect to chlorine conversion, it is not as efficient as is desired with respect to carbon monoxide. This is because the excess carbon monoxide in the original charge to the reactor passes through the reactor unreacted and is then incinerated. This represents a waste of this raw material. As a result, the net amount of carbon monoxide consumed by this process is substantially more than the amount actually necessary for the reaction to form phosgene.
A need therefore exists for a process for preparing phosgene by the reaction of chlorine with carbon monoxide which does not require a substantial net excess of either raw material.